1. Field of the Invention
The present invention relates to a measurement apparatus which measures spatial coherence, and an exposure apparatus including the same.
2. Description of the Related Art
In a lithography process for manufacturing devices such as a semiconductor device, mainly in photolithography, the NA of a projection optical system is increasing, and the wavelength of exposure light is shortening in an exposure apparatus. As the NA increases, the resolving power improves, but the depth of focus decreases. For this reason, from the viewpoint of forming finer patterns than ever before, increasing the NA of the projection optical system alone has proved to be insufficient to sustain stable mass production. Under the circumstances, so-called modified illumination methods which improve the resolution characteristics by optimizing an illumination optical system are attracting a great deal of attention.
There is a recent tendency to optimize not only the σ value, that is, the ratio between the NAs of the illumination optical system and projection optical system but also the effective light source shapes for individual original patterns. Examples of the modified illumination are annular illumination, quadrupole illumination, and dipole illumination.
However, the optical path changes upon changing the illumination mode, so spatial coherence on the original surface (in the object plane of the projection optical system) changes due to, for example, unevenness of an antireflection coating of an optical element which constitutes the illumination optical system or decentering of the optical element. Such a change in spatial coherence has a considerable influence on the quality of an image formed on the image plane of the projection optical system. It is therefore important to know spatial coherence and take it into consideration in designing an original and determining the effective light source distribution.
Known spatial coherence measurement methods are the following three methods. The first method is so-called Young interferometry or a double pinhole method (Joseph W. Goodman, “Statistical Optics”). The second method is shearing interferometry. The third method is a method of measuring spatial coherence based on a change in a certain pattern image.
FIG. 17 is a view showing the principle of Young interferometry as the first method. A plate 61 having two pinholes is irradiated by a light source 60, and the light beams from these two pinholes are made to interfere with each other on a screen 62 set behind the plate 61, thereby calculating spatial coherence based on the contrast of the obtained interference fringes. Japanese Patent Laid-Open No. 7-311094 describes an application example of the first method.
FIG. 18 is a view showing the principle of shearing interferometry as the second method. FIG. 18 schematically shows the structure of a Michelson interferometer. In the Michelson interferometer, when incident light 70 strikes a half mirror 71, it is split into a light beam 70a which travels toward a reference prism mirror 72 and a light beam 70b which travels toward a movable prism mirror 73. Light beams 70c and 70d reflected by the respective mirrors 72 and 73 return to the half mirror 71 and are superposed on each other to form interference fringes on a screen 74.
When the reference prism mirror 72 and the movable prism mirror 73 are placed such that two light beams which travel toward them have the same optical path length, and the movable prism mirror 73 moves in the Y-axis direction, the reflected light beam 70d also moves by the same distance and is superposed on the reflected light beam 70c. At this time, since the contrast of the interference fringes change in correspondence with spatial coherence, the spatial coherence can be measured by observing the moving distance of the movable prism mirror 73 in the Y-axis direction, and a change in the contrast of the interference fringes. Japanese Patent Laid-Open No. 9-33357 and Japanese Patent Publication No. 6-63868 describe application examples of the second method.
Japanese patent Laid-Open No. 10-260108 describe an example of the third method. According to Japanese Patent Laid-Open No. 10-260108, spatial coherence is measured by projecting a rhombic pattern and measuring the size of the projected image.
Unfortunately, Young interferometry as the first method has a demerit that the two pinholes need to be replaced a plurality of times to obtain spatial coherences at a plurality of points because the interval between these pinholes is fixed, taking a long period of time. Shearing interferometry as the second method has a demerit that not only a high-precision optical system is necessary but also it is hard to mount a shearing interferometer into an exposure apparatus which has significant spatial constraints because of its difficulty in downsizing. The third method is mainly used to measure the σ value (the ratio (NAill/NApl) between a numerical aperture NAill of an illumination optical system and a numerical aperture NApl of a projection optical system when circular illumination is performed by the illumination optical system). The third method has a demerit that it is necessary to measure the image size and compare it with a reference table provided in advance. The third method has other demerits that when an effective light source distribution is formed into a complicated shape instead of a circular shape, a table compatible with this shape is necessary, and that measuring the image size is insufficient to measure a complicated spatial coherence distribution.